1. Field of the Invention
The present invention relates to an alkali-free glass which is suitable as a substrate glass for various displays or photomasks and which is essentially free from alkali metal oxides.
2. Discussion of Background
Heretofore, the following properties have been required for a substrate glass for various displays, particularly for the one intended to form a thin film of a metal or oxide on its surface.
(1) It is essentially free from alkali metal oxides. If an alkali metal oxide is contained, the alkali metal ion tends to diffuse in the thin film, whereby the film properties will deteriorate.
(2) It has a high strain point, so that when it is exposed to a high temperature during the process for forming a thin film, deformation of the glass and shrinkage due to stabilization of the glass structure are suppressed to the minimum levels.
(3) It has adequate chemical durability against various reagents to be used for forming semiconductors. Particularly, it has durability against hydrofluoric acid used for etching SiO.sub.x or SiN.sub.x or against a buffered hydrofluoric acid (BHF) containing ammonium fluoride and hydrofluoric acid as the main components.
(4) It has no internal or surface defects (such as bubbles, striae, inclusions, pits or scratch marks).
At present, Corning code 7059 glass is widely employed as a substrate glass for various displays or photomasks. However, this glass has the following drawbacks.
(1) The strain point is as low as 593.degree. C., and preliminary heat treatment to reduce the shrinkage of glass has to be carried out prior to a process for preparing the displays.
(2) The amount of elution into hydrochloric acid used for etching a metal electrode or a transparent conductive film (such as ITO) is substantial, and the eluted substance tends to recrystallize during the process for preparing displays, whereby it becomes difficult to prepare the displays.
In addition to solving such problems, in recent years, it has been required to provide a glass having a small thermal expansion coefficient, which is strong against thermal shock in order to increase the temperature raising or lowering rate during the preparation of displays and thus to increase the production throughput.
Japanese Unexamined Patent Publication No. 160844/1989 (which corresponds to U.S. Pat. No. 4,824,808) discloses an alkali-free glass having a strain point of at least 625.degree. C., but the strain point does not exceed 650.degree. C.
Japanese Unexamined Patent Publication No. 160030/1992 discloses an alkali-free glass wherein the amounts of SrO and BaO are limited to certain levels, and some examples have a strain point of at least 650.degree. C. and a thermal expansion coefficient of from 30.times.10.sup.-7 to 40.times.10.sup.-7 /.degree.C. However, such examples have a liquidus temperature which is higher than a temperature at which the viscosity is 10.sup.4 poise which is an index for forming, and they are difficult to form by e.g. a float process.
Japanese Unexamined Patent Publication No. 263473/1994 (which corresponds to U.S. Pat. No. 5,374,595) discloses an alkali-free glass having a strain point exceeding 650.degree. C. and a thermal expansion coefficient of from 30.times.10.sup.-7 to 40.times.10.sup.-7 /.degree.C., but no evaluation has been made with respect to the resistance against BHF.
Japanese Unexamined Patent Publication No. 201041/1989 discloses an alkali-free glass having a strain point of at least 600.degree. C. and a thermal expansion coefficient of from 40.times.10.sup.-7 to 50.times.10.sup.-7 /.degree.C.